1. Field of the Invention
The present invention relates to copolymers and their use in radiation-sensitive layers of printing plates and the like; in particular, the invention relates to copolymers which increase the chemical and developer resistance of radiation-sensitive layers in positive working printing plates, printed circuit boards for integrated circuits and photomasks.
2. Background Information
Recently, the resistance of offset printing plates to solvents and common printing room chemicals such as plate cleaning agents or rubber blanket washing agents as well as to alcohol substitutes in the fountain water has had to meet ever increasing demands. Particularly in printing processes using UV-curing inks, where rinsing agents with a high content of esters, ethers or ketones are used, the chemical resistance of conventional positive working printing plates is no longer sufficient without special stabilizing processes. In order to improve the chemical resistance of positive working offset printing plates, three approaches have essentially been discussed:
1) In the most basic case, positive working offset printing plates comprise two components, namely a naphthoquinone diazide derivative (NQD derivative) and a novolak. One possibility is to chemically alter the NQD such that it is imparted an increased chemical and solvent resistance (e.g. U.S. Pat. No. 5,609,983). The disadvantage of this method is that the resistance of a printing plate produced therefrom cannot be increased indefinitely by increasing the NQD content since the NQD content has to lie in a certain range to make sense from a practical point of view.
2) Another approach is to thermally stabilize a conventional positive working printing plate (based on novolak and NQD). This can be carried out either by xe2x80x9cbakingxe2x80x9d or by means of the Pulsar(trademark) process. During baking, a completely developed printing form is heated to about 230xc2x0 C. for a few minutes causing the mostly phenolic binders to cross-link. This results in an extremely high chemical and mechanical resistance. In the other thermal process (i.e. the Pulsar(trademark) process), the developed printing plate is subjected to a short-time xe2x80x9cshock heatingxe2x80x9d to about 175xc2x0 C. This results in a slight cross-linking (and thus an increase in the chemical resistance) of the image areas. For the user, the two thermal stabilizing processes entail the important disadvantage that they require somewhat complicated apparatuses and are therefore quite costly.
3) A third option is the change of the binder content of a radiation-sensitive composition. For example, additives can be added to the formulation. However, in the case of commercially available additives, the amount to be added is often limited since these substances are not adapted to the composition and therefore most of the time affect the properties of the printing plate when added at too high a concentration.
Another frequently used method is the use of higher molecular novolaks; however, this leads to a deterioration of the photosensitivity of the composition which leads to a considerable loss of time for the user, particularly in the case of high numbers of prints or when so-called xe2x80x9cStep and Repeatxe2x80x9d exposure machines are used.
Furthermore, the properties of the binders in the photosensitive composition can be improved by way of chemical modification. Since such special binders with a high degree of chemical resistance are either not commercially available or very expensive (e.g. EP-A-0 737 896), it is often necessary that the manufacturers develop their own binders. In order to avoid high costs in the developing process, efforts have to be made to keep the synthetic requirements for preparing such tailor-made high-performance binders as simple as possible.
Therefore, despite intensive research in the field of chemically resistant binders for offset printing plates, the available approaches to solve the problem still leave some improvements to be desired, in particular with respect to binders that can be used more flexibly and are less expensive.
It is therefore an object of the present invention to provide inexpensive copolymers which significantly increase the chemical resistance of the radiation-sensitive layer in printing forms, printed circuit boards for integrated circuits and photomasks without suffering the disadvantages described above. Furthermore, development with conventional developers should be possible and a high degree of radiation sensitivity, good resolution and quick ink acceptance should be guaranteed. It is another object of the present invention to provide a process for the preparation of such copolymers.
The objects of this invention are achieved by a copolymer consisting of the units A, B and C wherein unit A is present in an amount of at least 5 to a maximum of 50 mol % and has the following formula 
wherein R1 and R4 are selected such that the homopolymer of A is alkali-soluble, unit B is present in an amount of 20 to 70 mol % and has the following formula 
wherein R2, R6 and R7 are selected such that the homopolymer of B has a high glass transition temperature , and unit C is present in an amount of 10 to 50 mol % and has the following formula 
wherein R3 and R5 are selected such that the homopolymer of C is water-soluble, with the proviso that unit C is different from unit A.